The background art will be described below with reference to the relevant drawings. Not every drawing shows all the reference numerals or symbols of the parts appearing in it, in which case reference is requested to be made to other drawings. For easy understanding, hatching may be omitted.
Typically, an image sensor converts light into electrical signals. To achieve that, as shown in a plan view in FIG. 22, an image sensor dve includes photodiodes pd for detecting light (broken lines g represent the borders between pixels). The larger the amount of light detected by the photodiodes pd is, the higher the sensitivity (performance) of the image sensor dve advantageously is.
However, there is a limit to enlarging the light-receiving portions of photodiodes pd in a small image sensor dve. This is the reason that various image sensors dve have been developed that are provided with microlenses that condense light on photodiodes pd. In FIG. 22, the microlenses are so formed that each of them is approximately as large as one pixel demarcated by broken lines g. Thus, the image sensor dve has cross-sections as shown in FIGS. 23A and 23B (cross-sectional views along lines P-P′ and Q-Q′ shown in FIG. 22). For the sake of convenience, on the pixel surface, the direction of the longer sides of the pixels is called the longer-side direction ld, and the direction of their shorter sides is called the shorter-side direction sd.
[First Fabrication Method]
The image sensor dve shown in FIGS. 22, 23A, and 23B is fabricated by the use of a mask mk having slits st as shown in FIG. 24 (the slit width is represented by d1). Now, this fabrication method will be described in detail with reference to FIGS. 25A to 25D. FIGS. 25A and 25C show a cross-section along line P-P′ shown in FIG. 22 (i.e., these are cross-sectional views of the image sensor dse along the longer-side direction ld, and FIGS. 25B and 25D show a cross-section along line Q-Q′ shown in FIG. 22 (i.e., these are cross-sectional views of the image sensor dse along the shorter-side direction sd.
As shown in FIGS. 25A and 25B, the image sensor dve includes a substrate unit scu including photodiodes pd. Over the substrate unit scu, a flattening film 131 is formed, and, further above, a lens material film 132 is formed as the material of microlenses ms (the flattening film 131 and the lens material film 132 are collectively called the microlens unit msu). The flattening film 131 is exposed through the mask mk and is then developed so that ditches (removed ditches) jd are formed in the flattening film 131. As shown in FIGS. 25C and 25D, the lens material film 132 having the removed ditches jd formed in it is then subjected to heat treatment and is thereby softened and melted. This causes the lens material film 132 to flow into the removed ditches jd, and thereby microlenses ms is formed.
However, with this fabrication method, in a case where the longer-side and shorter-side dimensions of each pixel differ, the microlenses ms come to have different curvatures in the longer-side and shorter-side directions ld and sd. This is because the flowing behavior of the lens material film 132 softened and melted depends on the distances over which it flows (i.e., the dimensions of the microlenses ms in the longer-side and shorter-side directions ld and sd).
When the microlenses ms are formed with different curvatures in the longer-side and shorter-side directions ld and sd, for example, a phenomenon as shown in FIGS. 26A and 26B (the optical path diagrams of the image sensor dse shown in FIGS. 23A and 23B) occurs. Superficially, while the light that passes through the curved surface of the microlenses ms corresponding to the longer-side direction ld is condensed on the photodiodes pd, the light that passes through the curved surface of the microlenses ms corresponding to the shorter-side direction sd is condensed in front of the light-receiving surface of the photodiodes pd. When this phenomenon occurs, the amount of light received by the photodiodes pd lowers, and thus the sensitivity of the image sensor dse lowers.
[Second Fabrication Method]
One way to overcome this problem is to adopt a fabrication method that uses a mask mk as shown in FIG. 27 (Patent Document 1). In this mask mk, the slit width d2 corresponding to the intervals between the shorter sides of the pixels is made comparatively large, no slit width is secured that corresponds to the intervals between the longer sides of the pixels, except that cuts ct are formed in the portions of the mask mk corresponding to the corners of the pixels. When the image sensor dse is fabricated by the use of this mask mk, the fabrication process is as shown in FIGS. 28A to 28D.
FIGS. 28A and 28C are cross-sectional views of the image sensor dse along the longer-side direction ld of the pixels, and FIGS. 28B and 28D are cross-sectional views of the image sensor dse along the shorter-side direction sd of the pixels.
As shown in FIG. 28A, the lens material film 132 is exposed to the light that has passed through the slit width d2 and is then developed so that removed ditches jd are formed in the lens material film 132 in the longer-side direction ld of the pixels. The lens material film 132 having the removed ditches jd formed in it is then subjected to heat treatment and is thereby softened and melted so that curved-surfaces are formed in the longer-side direction ld of the pixels (see FIG. 28C).
On the other hand, as shown in FIG. 28B, no removed ditches are formed in the lens material film 132 in the shorter-side direction sd. However, openings (unillustrated) corresponding to the cuts ct in the mask mk are formed in the lens material film 132, and the lens material film 132 flows into those openings; this produces curved surfaces in the shorter-side direction sd of the pixels (see FIG. 28D).
That is, according to the fabrication method of Patent Document 1, by adjusting the slit width d2 and the size of the cuts ct, the flowing behavior of the lens material film 132 in the longer-side and shorter-side directions ld and sd of the microlenses ms, and hence the curvature of the microlenses ms, is adjusted. However, since the slit width d2 is made comparatively large, the longer sides of the microlenses ms are shorter than the longer sides of the pixels. This produces, as shown in FIG. 28C, regions (non-lens regions na) where no microlenses ms are formed on the flattening film 131, and it is difficult to direct the light incident on these regions to the photodiodes pd. Consequently, an image sensor dse fabricated by this method cannot be said to have high sensitivity.
[Third Fabrication Method]
One way to fabricate an image sensor dse without producing non-lens regions is to adopt the fabrication method of Patent Document 2 shown in FIGS. 29A to 29G. According to this fabrication method, first, a resist film 133 having a ditch pattern pt is formed over a flattening film 131 (see FIG. 29A), and then etching is performed so that trenches dh corresponding to the ditch pattern pt are formed in the flattening film 131 (see FIG. 29B; the first patterning).
According to this fabrication method, thereafter, the resist film 133 is removed; then a lens material film 132 is formed over the flattening film 131 and is then exposed by the use of a mask mk having slits st whose width is larger than the width of the ditch pattern pt (it, the width of the trenches dh) (see FIG. 29C). Through development, removed ditches jd are formed in the lens material film 132 so as to correspond to the trenches dh in the flattening film 131 (see FIG. 29D; the second patterning).
What is to be noted here is that, because of the width (slit width) of the slits st, the removed ditches jd have a width larger than that of the trenches dh. Thus, between the bottom of the trenches dh and the surface of the lens material film 132, steps are left that are formed by the side walls of the trenches dh and the surface of the flattening film 131. Then, when the lens material film 132 is softened and melted, how the lens material film 132 in liquid phase flows depends on its surface tension and those steps. As a result, as shown in FIG. 29E, microlenses ms (main microlenses, which are convex lenses) are formed that have their edges on those steps.
Then, to prevent the trenches dh shown in FIG. 29E from producing non-lens regions, a lens material film 132 is formed anew and is then patterned (the third patterning) so that the lens material film 132 is left in the trenches dh (see FIG. 29F). When this lens material film 132 is softened and melted, as shown in FIG. 29G, microlenses ms (sub microlenses, which are concave lenses) are formed also in the trenches dh. Thus, according to the fabrication method of Patent Document 2, an image sensor dse free from non-lens regions is fabricated.
[Fourth Fabrication Method]
Another way to fabricate an image sensor dse without producing non-lens regions is to adopt a fabrication method employing, for example, a mask mk as shown in FIG. 30. In this mask mk, the slit width d4 corresponding to the intervals between the shorter sides of the pixels and the slit width d3 corresponding to the intervals between the longer sides of the pixels are made different (d3<d4). An image sensor dse fabricated by the use of this mask mk is as shown in FIGS. 31A to 31D. FIGS. 31A to 31D are drawn on the same principles of representation as FIGS. 28A to 28D.
As shown in FIGS. 31A and 31B, the lens material film 132 is exposed to the light that has passed through the slit widths d3 and d4 and is then developed so that removed ditches jd corresponding to the longer-side and shorter-side directions ld and sd of the pixels are formed in the lens material film 132. The lens material film 132 having the removed ditches jd formed in it is then subjected to heat treatment and is thereby softened and melted so that curved surfaces are formed in the longer-side and shorter-side directions ld and sd of the pixels (see FIGS. 31C and 31D).
Here, the width of the removed ditches jd formed to correspond to the slit width d3 is so adjusted that, in the longer-side direction ld, the edges of the microlenses ms are continuous, while making contact with the surface of the flattening film 131 (see FIG. 31C). On the other hand, the width of the removed ditches jd formed to correspond to the slit width d4 is so adjusted that, in the shorter-side direction sd, the edges of the microlenses ms are continuous, while being displaced above the surface of the flattening film 131 (see FIG. 31D). Thus, an image sensor dse is fabricated whose microlenses ms have different curvatures in the longer-side and shorter-side directions ld and sd and that is free from non-lens regions.
[Fifth Fabrication Method]
Also by the fabrication method of Patent Document 3, which uses a mask mk as shown in FIG. 32, an image sensor dse is fabricated that has microlenses ms as shown in FIGS. 31C and 31D. That is, an image sensor dse that is free from non-lens regions is fabricated.
[Sixth Fabrication Method]
One way to fabricate an image sensor dse that is free from non-lens regions and that allows fine adjustment of curvatures is to adopt the fabrication method shown in FIG. 33. FIGS. 33A, 33C, and 33E are cross-sectional views of the image sensor dse along the longer-side direction ld of the pixels, and FIGS. 33B, 33D, and 33F are cross-sectional views of the image sensor dse along the shorter-side direction sd of the pixels.
According to this fabrication method, after a photosensitive resist film 133 is formed over the lens material film 132, the lens material film 132 is exposed through the mask shown in FIG. 30, and is then subjected to heat treatment A(see FIGS. 33A and 33B). Thus, as shown in FIGS. 33C and 33D, the resist film 133 is formed into the shape of microlenses. Then, dry etching is performed, with the etching rates for the resist film 133 and the lens material film 132 so adjusted that the selection ratio is approximately “1”. This causes the microlens ms shape of the resist film 133 to be transferred to the lens material film 132.
According to this fabrication method, by setting the selection ratio (the ratio between the etching rates for the resist film 133 and the lens material film 132) slightly greater than “1”, it is possible to transfer the shape of microlenses ms to the lens material film 132 while varying the curvature of the microlens ms shape of the resist film 133 (see FIGS. 33E and 33F). Thus, also according to this fabrication method, an image sensor dse is fabricated whose microlenses ms have different curvatures in the longer-side and shorter-side directions ld and sd and that is free from a non-lens regions.    [Patent Publication 1] JP-A-H7-113983 (the second fabrication method above)    [Patent Publication 2] JP-A-2000-260970 (the third fabrication method above)    [Patent Publication 3] JP-A-H8-288481 (the fifth fabrication method above)